Mechanical actuators with pistons are widely used in industry for moving parts or components of machinery to carry out various functions. Actuators are used in assembly lines or industrial processes to control valves, or to operate equipment. Actuators usually operate using pneumatic or low pressure hydraulic fluid to create a force, linear or rotary, to move a component or piece of machinery.
Pneumatic pistons or actuators are of two basic types:
A. Bellows. These typically are hollow and consist of preformed rubber which extends and contracts in a linear manner by an "accordion" mechanism extending or collapsing the elastomer. To avoid radial bulging, the rubber must be very heavy, horizontal movement must be very short in relation to the radial dimension of the accordion shape, and pneumatic pressure must be sufficiently low so as not to rupture the rubber. Bellows type pistons are useful primarily for short thrust, low pressure movements such as switch or brake activation. Typical maximum working pressures of bellows type pistons are limited to about 20 psig. PA1 B. Solid tube pistons. These actuators typically comprise a solid piston sliding within a hollow solid (usually metal) tube. Solid tube pistons typically operate at working pressures in the range of about 80 psig. To contain the required pneumatic force on the piston, one or more rubber air seals enclose the circumference of the piston and thereby contain the air. The air seals are similar to piston rings in an internal combustion engine. Typically, since the piston moves along the axis of the interior of the tubular cylinder, a linear force is generated. The term "actuator" is often applied in situations where a rotary (torque) force is to be generated. In the case of mechanical actuators, the rotational force is usually obtained by utilizing a rack and pinion arrangement within the cylinder. The rack is attached to the piston and the pinion exits the cylinder radially. This requires a seal (an O-ring, for example) to contain the air pressure. Various types of actuators are available, for example, double action and spring return. PA1 A. The principle of physics that when pressure is applied to the inside surfaces of an "elastomer bag" of any shape (for example, an elongated balloon) the pressure will tend to force the bag into a spheroid shape. Thus the pressure attempts to equalize itself within the confines of the volume. This is described herein as "equalizing pressure". PA1 B. Restraining radial expansion of an elastomer bag by a series of two opposed diagonal windings for which the angle of the crossing points changes to allow some lengthening of the tube until a maximum angle change occurs. This is described as "radial constraint".
The sliding piston in a fixed cylinder is commonly used for applications such as valve stem rotation. The inherent problem with this type is that they are expensive to manufacture and have wear and friction problems associated with the necessity for sliding seals on the pistons. Contaminated air can significantly shorten the life of the seals, and the design of such actuators does not permit economical serviceability. Some applications therefore require the air to be filtered or otherwise treated to prolong actuator service life.
Other linear movement mechanisms exist which comprise a tube that stretches in a linear manner, such as for air ducting used in ventilation systems. These stretchable tubular mechanisms include plastic tubing with embedded coiled wire which allows horizontal stretch of the tubing. The coiled wire provides radial strength. There is an inherent problem with such tubes. When a high pneumatic pressure is applied to the tube, it tends to turn and cause localized bulging. Such tubes with internal or embedded coils are thus suitable only for very low pressure applications.
Various inventors have attempted to solve the problems inherent in the designs of these two types of actuators by using a sealed rubber tube (air bag) and restraining its radial expansion by various means other than a bellows. These systems generally involve surrounding the rubber tube with an outer tube having helical wires. This allows the outside tube to stretch without bulging. Another method utilizes a second outside tube with compensating pneumatic pressure. These systems generally shorten the available stroke of the actuator relative to its length and also set up counteracting forces which significantly decrease the mechanical efficiency of the expanding inner tube.
Actuators usually employ one of two methods for activation:
A number of patents have been issued over the years disclosing various devices that employ one or the other, or both, of principles A and B above.
Beullens--U.S. Pat. No. 4,841,845
Beullens utilizes the equalizing pressure principle. This is demonstrated by the description of FIGS. 1 and 2 as being in the inactive position and FIG. 3 as being in the active position. Column 4, paragraph 40, discloses that "the working points . . . are pulled towards one another". The purpose of the spiral wires in Beullens appears to be not only to stop the device from "blowing up" but also to redirect the radial force to a horizontal sucking force when maximum radial size is reached.
The device comprises on the one hand at least one tightly-sealable chamber, which is restricted by a wall made from a partially distortable material, and on the other hand flexible, approximately unstretchable spiral-wound filaments which extend substantially next to one another at least about said wall, whereby part of said filaments are wound rightwards and another part thereof leftwards, and this in such a way that two arbitrary crossing filaments may undergo some angular displacement relative to one another, and the one end of each said filaments on the one side of said chamber is fixed relative to a working point, and the other end thereof on the opposite side of said chamber is fixed relative to another working point, and whereby further at least one feed opening is provided in said chamber, wherethrough a pressurized gas or liquid may be fed and said wall is distortable at least along one direction cross-wise to the line joining both said working points, in such a way that by regulating the gas or liquid pressure inside the chamber, a relative displacement of said working points occurs.
Negishi--U.S. Pat. No. 5,201,262
Negishi utilizes the radial constraint principle. The actuator of Negishi includes an elastic member extensible in axial directions when a pressurized fluid is supplied into the elastic member, and a guiding device arranged inwardly of the elastic member and permitting the elastic member to move in the axial directions but restraining the elastic member from moving in directions intersecting the axial directions. The actuator is of an air-bag type so that energy of the pressurized fluid can be converted into mechanical movement with high efficiency. The actuator moves only in axial directions without expanding in radial directions, so that a space occupied by the actuator in operation is little. Due to the restrictions of angle change of the "reinforcing braided structure", there is limited travel of this actuator in relation to its length. This limits its application. The other "embodiment" (FIG. 3a) is the addition of a return spring outside the actuator.
Negishi--U.S. Pat. No. 5,158,005
The device disclosed by Negishi in this patent is very similar to the device in his U.S. Pat. No. 5,201,262, except that the guiding tube is now outside instead of inside. The actuator of this patent includes an elastic member extensible in axial directions when a pressurized fluid is supplied into the elastic member, and a guiding device arranged outwardly of the elastic member and permitting the elastic member to move in the axial directions, but restraining the elastic member from moving in directions intersecting the axial directions. The actuator is of an air-bag type so that energy of the pressurized fluid can be converted into mechanical movement with high efficiency. The actuator moves only in axial directions without expanding in radial directions, so that the actuator takes up little space in operation. The telescopic tube appears to be used not to prevent expansion of the elastomer (this is done by the braided structure) but to keep the piston pointed in the same direction. If the braided structure were not there, the elastomer would abrade against and pinch against the telescopic tube. There is limited travel on this piston.
Negishi--U.S. Pat. No. 5,067,390
Negishi, in this case, employs a combination of the equalizing pressure and radial constraint principles, whereby there are two concentric pressure tubes. The double-acting actuator of U.S. Pat. No. 5,067,390 includes a tubular body made of an elastic material, with a first reinforcing braided structure surrounding it. A second tubular body made of an elastic material surrounds the reinforced braided structure to form a space outwardly. A second reinforcing braided structure surrounds the second tubular body. The actuator further includes closure members for closing and joining ends of the first and second tubular bodies and reinforcing braided structures, and guiding device for permitting axial movements of the first and second tubular bodies but restraining lateral movements thereof. The first and second reinforcing braided structures are so constructed that initial braided angles thereof permit of the first braided structure elongating and permit of the second braided structure contracting when the pressurized fluid is supplied into the first and second tubular bodies. The fluid pressure is varied between the tubes so that the outside tube at one point has higher pressure than the inside tube and thus restrains radial expansion, directing the force to horizontal thrust. This device also has limited movement.
Sakaguchi--U.S. Pat. No. 4,860,639
Sakaguchi discloses a classic example of the equalizing pressure principle. The actuator of Sakaguchi includes a tubular body made of a rubber-like elastic material and a braided structure made of organic or inorganic high-tensile-strength fibers reinforcing an outside of the tubular body. Closure members sealingly close ends of the tubular body; at least one of the closure members has a fluid connecting passage. The tubular body deforms to expand its diameter when pressurized fluid is introduced through the connecting passage to cause contractive force in the longitudinal direction. Contraction-detecting strain gauges at one closure member provide signals corresponding to the contractive force of the actuator.
Takagi--U.S. Pat. No. 4,615,260
This device also operates according to the equalizing pressure principle with modifications to improve and decrease fatigue. Takagi discloses a pneumatic actuator including an elastic tubular body, closure members sealingly closing its ends and a braided structure made of braided cords reinforcing the tubular body. The braided structure is expanded in its radial direction and simultaneously contracted in its axial direction together with the tubular body when pressurized fluid is supplied into the tubular body. According to the invention the braided cords of the braided structure comprise monofilaments, each having a smoothly rounded outer surface of a large radius of curvature. A protective layer may be provided between the tubular body and the braided structure or a filler such as an incompressible fluid substance having no constant shape is provided in the tubular body, or diameters of both ends of the braided structure and braided angles at both the ends are made larger than those at a substantially mid-portion of the braided structure. The actuator according to the invention decreases damage of the tubular body to elongate its service life and exhibits an improved contacting performance and high fatigue strength and can greatly save air consumption to eliminate the disadvantage of much air consumption of the air-bag type actuator without adversely affecting its advantages.
Wang--U.S. Pat. No. 4,833,973
The fluid pressure actuated assembly disclosed in Wang includes a casing made of a flexible resilient material, such as rubber or polyurethane, a coiled tension spring sleeved on the casing for biasing the casing to move toward a retracted position, and a coiled spacing spring interposed between the tension spring and the casing for preventing any wall of the casing from being clamped between any two adjacent turns of the tension spring. When a compressed fluid is applied to the interior of the casing, the casing extends. This uses the return spring for radial restraint, but adds a spacing spring in between to prevent the flexible material from pinching between the turns of the return spring.
Paynter--U.S. Pat. No. 4,108,050
Paynter discloses a method of creating a torque by pressurizing the inside of a tube having preformed spiral spring wires (helically shaped) on the outside. The expansion pressure forces the wires to straighten (ie. lose their spiral) and thus turn one end of the device.
Vergenet--U.S. Pat. No. 4,008,008
The invention, among other things, provides a pump adapted for the intake and delivery of liquid such as water in wells or relatively deep bodies of water. The pump comprises a rigid-walled chamber, adapted to be immersed in the liquid to be sucked in. The rigid-walled chamber has an intake valve and a delivery valve interposed between the rigid-walled chamber and a delivery tube. The pump is characterized in that it comprises, accommodated in the rigid-walled chamber, a resiliently deformable chamber associated with means for controlling, at least in one direction, alternate deformations of the chamber by expansion and retraction. This is a device for a submersible pump (well pump, for example). There is a deformable plunger on the end of the handle at the top to increase the pressure exerted on the water in the well, forcing the water up a tube.
Larsson--U.S. Pat. No. 4,777,868
Larsson discloses a flexible actuator, comprising at least a pressure tube, which is axially extendable and/or contractible under influence of a pressure fluid. The object of the invention is to provide a flexible actuator, which can perform straight axial movements as well as curved movements in one or more planes and which can also operate at very high pressures. These objects have been achieved by the fact that the tube (12) with the exception of its end, connection or attachment parts (13) is corrugated and that at least the portions (10) of the corrugated tube, which are located between its outward projecting folds (9), are equipped with means (8) of a material which is inextensible as compared to the material of the tube, and arranged substantially to prevent a radial expansion and/or contraction of the tube in said portions (10). This is effectively a very long bellows type with strengthening in the folds of the bellows to prevent bulging. He has claimed many variations to prevent the bulging, but all rely basically on the bellows idea and strengthening with helical wire reinforcing.
Price--U.S. Pat. No. 4,006,669
Price discloses a fluid pressure activated piston slidably carried in a fluid pressure actuated cylinder which, in turn, is slidably carried in a fixed carrier. Movement of the cylinder is resisted by a deformable tube frictionally engaged with a fixed circular member. A predetermined fluid pressure acting across a differential area wall portion of the cylinder generates a force overcoming the frictional resistance of the deformable tube engaged with the fixed circular member thereby advancing the cylinder in the direction of movement of the pressurized piston. The output force of the piston is substantially unaffected by the force imposed on the cylinder. This is a very complicated device to be used for aircraft brake actuation. The only flexible material appears to be a radially deformable member inside the cylinder to alter the movements.